PDF vouchers plugin will only work with downloadable products as it needs to allow download the vouchers once the order get completed.
We have checked the plugin from the link that you have provided and it seems that free version of the plugin doesn’t supports downloadable products. Please check the screenshot: https://prnt.sc/ixy1yc
It’s only supported in WCFM Ultimate plugin.
Can you please provide WCFM Ultimate plugin so, we can add compatibility.
Thanks in advance.